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2 I 4 CHAPTER 5 PHYSIOLOGICAL AND TOXICOLOGICAL CONSIDERATIONS
FIGURE 5.23 Airstream velocity profiles through a bronchial bifurcation. Shear forces along the
medial bronchial wall cause flow distortions in the daughter tubes during inspiration. Bimodal velocity
profiles generated in the parent tube during expiration are also caused by shear along the daughter tube
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medial walls. Modified from Scherer and Haselton.
where c = solute concentration; x = direction of airflow; y, z ~ transverse di-
rections to flow; and K = dispersion constant. K depends on the molecular
diffusion coefficient, D ah, where Pick's law defines mass transport by molecu-
lar diffusion as
where U = convective velocity. Pulmonary airways rely solely on molecular
diffusion for mass transport.
5.2.4 Mucociliary Clearance
Mucus gel floating on airway periciliary fluid becomes contaminated by atmo-
spheric contaminants deposited onto the air-mucus interface during respira-
tion. Deposition generally traps these materials, especially particulates, in the
mucus gel and prevents them from being transported further by the airstream.
Merely trapping these materials, however, serves little purpose because they
would diffuse through the periciliary fluid to enter the epithelia and blood-
